Plug-in hybrid electric vehicles (“PHEVs”) and pure battery electric vehicles (as a group plug-in electric vehicles, PEVs) promise to facilitate a transportation future that is less dependent on liquid fossil fuels. However, as PEV market penetration increases, vehicle charging could strain aging power delivery infrastructure. A number of recent papers have shown that increases in PEV charging could have detrimental impacts on medium and low voltage distribution infrastructure, particularly where PEV adoption is highly clustered. With mass-produced PEVs coming to market and a range of charging standards (AC Levels 1-3) established, it is increasingly important to understand and mitigate negative impacts that PEV charging might have on distribution system components, such as underground cables and transformers.
Some charging of PEVs is envisioned to occur at workplaces, shopping centers, etc., where the power distribution is already sufficient to support commercial endeavors. However, it is more likely that PEV charging will primarily occur at a person's home where the existing power distribution system has been designed for residential scale service, which is typically limited by 15-25 kVA transformers and underground cables that have capacities on the order of 100 kVA. At Level-1 charging rates (˜1.4 kW), electric vehicle charging can double the electricity use of an average U.S. residence (from 1.2 kW to 2.6 kW). At Level-2 charging rates (˜7 kW), residential loads increase even more dramatically. This additional load can have substantial detrimental effects on residential distribution infrastructure, particularly transformers and underground cables, even under moderate PEV penetration scenarios. For example, transformers, substations, and underground cables can age rapidly if operated beyond their specified thermal limits due to the additional power draw by large loads.
Implementing effective charge management (CM, also known as smart charging) methods is one step to facilitate the smooth integration of PEVs. Several previous studies show that with effective CM schemes it is possible to support large numbers of electric vehicles even with constrained electric power infrastructure. In many cases it is also possible for PEVs to not only avoid negative impacts on the power grid, but also to provide grid services, through Vehicle-to-Grid (V2G) technology.
The CM and V2G control schemes that have been proposed in the literature, or in industry, generally fall into one or both of the following categories:
1. Centralized optimization or control methods in which each vehicle submits information to a central authority, which in turn solves an optimization problem that produces a charging schedule for each vehicle.
2. Decentralized methods, in which either utilities set a pricing scheme (e.g., a fixed time-of-use price) and vehicles self-schedule based on those prices, or a market-based scheme is used to generate prices to which vehicle charge management devices respond.
These two approaches have a variety of advantages and disadvantages.
Centralized schemes have the advantage that they can, under some conditions, produce economically optimal outcomes by minimizing costs and avoiding constraint violations in the distribution system. However, optimization/control methods require that vehicle owners provide information (e.g., willingness to pay or anticipated departure times) to a central authority and require that the vehicle owner give up at least some autonomy over the charging of their PEV. While the load serving entity would likely compensate the vehicle owner for this loss of control with a reduced rate for electric energy, this loss of autonomy could be an impediment to adoption of CM schemes. In addition, vehicle owners are unlikely to know in advance their exact travel schedule, which complicates the problem.
The use of dynamic pricing schemes has been suggested to mitigate the detrimental effects of PEV charging. However, in order for a dynamic pricing scheme to mitigate localized transformer or cable overloading, utilities must install infrastructure that:
(1) determines current capacity and demand; (2) adapts local rates based on this capacity and demand; and (3) relays this rate information to each customer. Furthermore, price-based schemes will require a consumer charging system that: (1) can communicate with the power distribution system; and (2) enables customers to choose to charge based on fluctuating prices or at least have technology installed for making charging decisions. Note that a price-based approach typically requires the power distribution system to know specific information about specific customers, which exacerbates existing concerns about data privacy and security in a Smart Grid environment.
Simple time-of-use pricing schemes, such as a reduction in rate for nighttime charging, do not have these disadvantages as owners have flexibility in choosing how they will respond to the change in prices. However, very simple price-based schemes are unlikely to produce optimal outcomes in terms of avoiding overloads in the distribution network, or minimizing costs. In fact, such time-differentiated pricing could produce new load peaks that increase, rather than decrease aging in the distribution infrastructure.
The stochastic nature of charging behavior is particularly important to highlight. PEV arrival and departure times vary substantially between owners, days, and within days. Feeder load variability and uncertainty will grow even further with an increase in distributed renewable generation. Vehicle CM schemes that do not adapt well to such uncertainty are unlikely to be successful.
Additionally, other large loads, such as, for example, air conditioning systems, tax the power distribution grid in similar fashion. Accordingly, there is a need for a charge management scheme that enhances the equitable distribution of power to customers, improves the optimal use of the available supply of power, and offers better privacy to customers.